It is known that a latent electrostatic image can be developed with toner particles dispersed in an insulating non-polar liquid. Such dispersed materials are known as liquid toners. A latent electrostatic image may be produced by providing a photoconductive layer with a uniform electrostatic charge and subsequently discharging the electrostatic charge by exposing it to a modulated beam of radiant energy. After the latent electrostatic image has been formed, the image is developed by colored toner particles dispersed in a non-polar liquid. The image may then be transferred to a receiver sheet.
Liquid toners comprise a thermoplastic resin and a dispersant non-polar liquid. Generally, a suitable colorant, such as a dye or a pigment, is also present. Since the formation of proper images depends on the difference of the charge between the liquid developer and the latent electrostatic image to be developed it has been found desirable to add a charge director compound and preferably other adjuvants which increase the magnitude of charge to the liquid toner comprising the thermoplastic resin, the non-polar liquid and the colorant.
The colored toner particles are dispersed in a non-polar liquid which generally has a high volume resistivity in excess of 10E+9 ohm-cm, a low dielectric constant, and a high vapor pressure. Once the image is printed on the output media (e.g. paper, transparency, etc.), due to high volatility of the liquid, the toner image on the output media dries readily. This dry image is resistant to abrasive and shear failure due to toner polymer returning back to its virgin resin properties.
Use of a high volatility liquid has several disadvantages. Both the printing mechanism and the printed output media become prime sources for contamination of indoor air leading to a variety of chemically induced ailments in humans. This has forced us to use low volatility liquids. The low volatility liquids do not leave the printed image readily and drastically impair the fixability (toner fusing to output media), leaving the image exposed to easy removal from the output media. The invention claimed herein separates this resin/liquid interaction, and shows the materials and a process designed such that even with the liquid present in the toner image the image is as tough as the virgin resin. The resins of this invention do not plasticize substantially in the liquid they are dispersed in. Typically, the resin families most commonly used in the experiments have been terpolymers of styrene/acrylate/methacrylic acid monomers.
U.S. Pat. No. 5,019,477 to Felder is to a liquid toner comparable to this invention. At column 4, beginning at line 42 it states that its toner solids are substantially insoluble in the carrier liquid. The resin polymers of this patent are not particularly similar to the terpolymer of this invention. This patent teaches a resin composition as a mixture of: i) copolymer of ethylene and acrylic acid (e.g. Nucrel) ii) copolymer of styrene (or vinyl toluene) and an acrylate. This invention differs by: i) its use of an acid in a terpolymer, ii) its use of very low volatility white oil, and iii) the absence of polyethylene.
U.S. Pat. No. 5,116,705 to Materozzi similarly discloses resins insoluble in the vehicle.
U.S. Pat. No. 3,668,127 to Machida et al is to a liquid toner having resin coated pigment in which the resin may be an acid containing terpolymer.
U.S. Pat. No. 3,890,240 to Hochberg teaches a composition of liquid toner developers which comprise: 1) volatile hydrocarbon solvent (e.g. Isopar brand), 2) dissolved terpolymer of vinyltoluene, butyl methacrylate, lauryl methacrylate, 3) carbon black & colorants, and 4) metal soap. This invention teaches a different liquid toner developer system as follows: 1) use of non volatile white oil (e.g. Marcol 82 brand), 2) Non swelling (non soluble) resin being a terpolymer of styrene, butyl acrylate, and methacrylic acid. The last component has no analog in U.S. Pat. No. 3,890,240.
In U.S. Pat. No. 4,156,034 to Mukoh et al, the liquid toner materials may be similar in that: compound (iii) col. 1.14, line 45 could be acrylic acid, and compound (iv) col. 2, line 65 could be butyl acrylate, but differs from our invention in that: a) compound (ii)is para alkylstyrene with a minimum alkyl chain length specified; while this invention employs styrene, b) this invention employs highly viscous white oil, c) and the examples of Mukoh et al never indicate compound (i) col. 2, line 16 to be a direct acid.
U.S. Pat. No. 4,814,251 to Igoe discloses liquid toners comprising of: i) vinyl toluene acrylic terpolymer (3 to 7 percent wt.) ii) acrylic copolymer (5 to 20% wt.) iii) pigment (10 to 18% wt.) and iv) high volatility isoparaffin solvent (60 to 80% w/w). This invention differs in that: i) this invention employs a very low volatility white oil, ii) this invention employs an acid together with styrene and butyl acrylate in a terpolymer, and iii) does not employ vinyl toluene.
Such resin modifications as investigated in inventions mentioned above may provide adequate function in highly volatile carrier liquid, but would not achieve the critically needed toner particle charging and image fixability in liquid of low volatility. Our toner formulations utilizing a highly viscous carrier liquid such as Marcol 82 brand mineral oil accomplish both.